Kwesi owns two fifths of a
housing estate - His brother
Kwame owns one third of the estate
and theire sister Abena owns the
remainder
i) What fraction of the housing estate
do the brothers kwame and kwesi
ii) What fraction of the housing estate
does Abena own?

You would start by finding the least common denominator of the two fractions. In this case it is 15. You would then convert your fractions from 2/5 to 6/15 by multiplying your numerator AND denominator by whatever number gets you to the the least common denominator you found previously. Do this again with 1/3 gets you 5/15. You then add 6/15 and 5/15 = 11/15. You know that Abena owns the rest so 15/15 - 11/15 = 4/15. Abena owns 4/15 of the estate

When it comes to undefined terms Mathematicians uses a general description for their meaning rather than a formal definition; So therefore, a line is an undefined term because it does not have a formal definition.